Borosilicate opal glasses

ABSTRACT

THIS INVENTION RELATES TO THE PRODUCTION OF BOROSILICATE OPAL GLASSES WHEREIN THE OPAL PHASE IS READILY ATTACHED BY WATER AND DETERGENTS. IN THE GLASSES OF THE PRESENT INVENTION, THE OPAL PHASE IS MADE DISCONTINUOUS, I.E., THE PARTICLES THEREOF COMMONLY EXHIBIT A SPHERICAL CONFIGURATION, SUCH THAT DEEP LEACHING IS EXHIBITED. THE GLASSES CONSISTS ESSENTIALLY, IN WEIGHT PERCENT ON THE OXIDE BASIS, OF 0.5-2.5% LI2O, 7-10% ZNO, 11-14% B2O3, AND 71-76% SIO2.

March 27, 1973 PLANNER)! ETAL 3,723,144

BOROSILICATE OPAL GLA SSES Filed May 10, 1971 v e Sheets-Sheet 1.

a rf IN [/5 N TORS.

James E. F lannry Jacques 6. Lemoine AT TORNEY' March 1973 E, FLANNERYETAL 3,723,144

BOROSILICATE OPAL GLASSES 6 Sheets- INVENTORSL: James E. F IanheryJacques .G. Lemoine BY AT ORNE March 27, 1973 E, FLANNERY ET ALBOROSILICATE OPAL 'GLASSES 6 Sheets-Sheet 3' Filed May 10, 1971INVENTU'RSI. James E F lannery Jacques 6. Lemaine 1 7d! 27. 1973 J, E.FLANNERY ETAL $723,144

. BOROSILICATE OPAL GLASSES I Filed lay 10, 1971 6 Sheets-Sheet 4INVENTORS. James E. F Iannery Jacques G. Lemaine BY A ORNEY March- 27,1973 J. E. FLANNERY ET AL BOROSILICATE OPAL GLAS SES 6 Sheets-Sheet 5Filed May 10, 1971 ine ORNEY March 27, 1973 J- E. FLANNERY 3,723,144

BOROSILICATE OPAL GLAS SE5 6 Sheets-Sheet 6 Filed May 10, 1971 m sm R!-W F N. WE w James Jacques AT ORNEY United States Patent 3,723,144BOROSILICATE OPAL GLASSES James E. Flannery, Corning, N.Y., and JacquesG. Lemoine, Fontainebleau, France, assignors to Corning Glass Works,Corning, NY.

Filed May 10, 1971, Ser. No. 141,765 Int. Cl. C03c 3/08 U.S. Cl. 106-542 Claims ABSTRACT OF THE DISCLOSURE This invention relates to theproduction of borosilicate opal glasses wherein the opal phase isreadily attacked by water and detergents. In the glasses of the presentinvention, the opal phase is made discontinuous, i.e., the particlesthereof commonly exhibit a spherical configuration, such that deepleaching is inhibited. The glasses consist essentially, in weightpercent on the oxide basis, of 0.5-2.5% Li O, 710% ZnO, 11-14% B 0 and71-76% SiO CROSS REFERENCE TO RELATED APPLICATION U.S. application Ser.No. 94,834, filed Dec. 3, 1970 in the names of Gerald B. Carrier andJames E. Flannery.

A translucent or almost opaque white glass can be produced through theinclusion therein of colorless, nonmetallic crystalline or amorphousparticles. These particles, having a dilferent index of refraction fromthat of the base glass, scatter the light within the body of the glassand diffuse the transmitted light. Such glasses have been referred to asopals and have seen such diversitied applications as tableware, culinaryware, lamp globes, and wall paneling.

Most frequently, the opacity exhibited by opal glasses is dependent upona phase separation occurring within the body of the glass wherein alight-diffusing phase is uniformly precipitated throughout a transparentglassy matrix. The light diffusing effect results from the fact that theseparated phase, whether it be amorphous, crystalline, or only voids,has an index of refraction different from that of the matrix glass suchthat light scattering with consequent loss of transparency occurs.

The opacifying phase may be relatively continuous through all or aportion of the glass body. Conversely, it can be particulate orotherwise relatively discontinuous. Where the separated phase iscontinuous within itself, deep leaching can occur. However, if theprecipitated phase can be made discontinuous, then paths for leachingwill not be available and only those portions of the soluble phase whichare exposed at the surface of the glass will be dissolved. This, ofcourse, presupposes that the baseglass is relatively inert to theattacking medium.

In the manufacture of opal glasses, an opalizing agent is included inthe glass batch which will form a compound that is soluble in the 'glassmelt but which will precipitate therefrom during the cooling of the meltto an amorphous body or upon a subsequent heat treatment thereof. Opalglasses produced from conventional soda lime glass composition were Wellknown to the art and commonly utilized metal halide, sulfate, orphosphate salts as opalizers. Nevertheless, the high coefficients ofthermal expansion exhibited by these glasses, viz., up to 90X C. overthe range 25 -300 C., foreclosed their use where reasonable resistanceto thermal shock was required, as, for example, in ovenware. In view ofthis deficiency, opal glasses having a borosilicate base compositionwere developed. These glasses demonstrate coefficients of thermalexpansion over the range 25 300 C. between about ZS-SOX 10 C. and,therefore, exhibit sufficient thermal shock resistance for use inglassware applications involving high temperature gradients resultingfrom zone heating or thermal cycling.

U.S. Pat. No. 3,275,492 describes one group of such borosilicate-typeglasses having good resistance to thermal shock within a broad range ofopal densities. Opalizing agents for borosilicate-type glasses includeZnO, MgO, CaO, BaO, NiO, CoO, MnO, and C with, optionally, suchsecondary opalizers as the halides, phosphates, or sulfates. Theseglasses exhibit good melting and forming properties but, unfortunately,are subject to chemical attack from certain commonplace solutions. Forexample, tableware and culinary ware made therefrom manifest surfaceattack after being in contact with hot detergent solutions such as areexperienced in dishwashers. This chemical attack all too soon leads tosurface roughness which, in itself, is aesthetically unpleasing andfurther entrains staining from food coming into contact therewith.

A study of this lack of chemical durability has pointed out twoprincipal causes therefor: First, and most impor tant, the opal phase issoft and highly susceptible to chemical attack; and, Second, the opalphase is relatively continuous in the glassy matrix. Thus, the opalphase s much less resistant to chemical attack than the surroundingmatrix glass. Therefore, inasmuch as this separated phase is highlysoluble in the contacting detergent solution, continuity of this phasewithin the glass body provides leaching paths which permit the attackingsolution to migrate deep into the body. Such deep penetration andsolution can obviously be a serious problem in such applications asovenware where food entering the resultant voids may cause discolorationor staining. Where, however, the precipitated soluble phase is developedin the form of individual droplets with few or no interconnections, anattacking medium can leach only those droplets near the surface of thebody and deep penetration resulting from chemical attack can beobviated.

In U.S. application Ser. No. 94,834, filed Dec. 3, 1970 in the names ofGerald B. Carrier and James E. Flannery, one method for securingdiscontinuity in the separated phase is disclosed. That applicationdescribes the addition of minor amounts of M00 and/or W0 and/ or As O tothe base glass batch consisting of 70-80% SiO and 53-15% B 0 withoptional additions of alkali metal and bivalent metal oxides. Thepreferred opal glasses contain 1-6% of alkali metal oxides, 3-4% totalof the bivalent metal oxides ZnO, CaO, and MgO, and 0.2-3% total of M00W0 and As O Electron micrograph comparisons of the internal structuresof conventional borosilicate opal glasses and the opal glasses of thatinvention showed a significant difference in the microstructure of theprecipitated phase. In the conventional opal glass, the opacifying phaseappeared as a series of interconnecting, irregularly-shaped dropletsscattered throughout the cross section. In contrast, the separated phasein the new glasses appeared as individual, disconnected droplets.

The mechanism through which the M00 W0 and/ or As O operate to achievethe highly discontinuous opal phase could not be explained, it beingtheorized that those metal oxides had some effect upon the surfacetensions of the matrix glass and/or the opacifying phase which promotedthe development of that phase in the form of small discontinuousdroplets. Although the resulting improvement in chemical durabilityimparted to the opal glasses through the additions of M00 W0 and/orAS203 thereto was very real, such additions add materialy to the costthereof and, particularly with respect to AS203, comprise a toxic hazardin the mixing and melting of the batch ingredients. Therefore, meanswere sought to accomplish the same function but without the use of thesemetal oxides.

Further, the opal products of that invention exhibited a soft whiteappearance whereas, for such applications as dinnerware and culinaryware, a bright white body is seemingly highly desirable to the user.

Therefore, the principal object of the instant invention is to producean opal borosilicate glass exhibiting a very bright white appearancewith improved chemical durability, the improved chemical durabilitybeing due to the opacifying phase being present in the form of verysmall, discontinuous, spherically-shaped droplets, wherein the glassbatch consists essentially only of Li O, B ZnO, and SiO Other objectswill become apparent from the following description and the appendedelectron micrographs wherein:

FIG. 1 is a replica electron micrograph illustrating the cross-sectionalmicrostructure of a conventional opal borosilicate glass;

FIG. 2 is a replica electron micrograph depicting the cross-sectionalmicrostructure of an opal borosilicate glass containing M00 made inaccordance with the process described in application Ser. No. 94,834;

FIGS. 3-6 are replica electron micrographs demonstrating thecross-sectional microstructure of opal borosilicate glasses havingcompositions within the parameters of the instant invention; and

FIGS. 7-11 are replica electron micrographs illustrattogether, will beconverted to the desired oxide in the proper proportions. The batchingredients in Table I were melted in closed platinum crucibles at aboutl450- 1600 C. for about 16 hours. The melt was poured onto a steel plateto form a patty about in diameter and in thickness. No fining agent wasemployed but, where desired, conventional fining agents can be utilized.The patty was immediately transferred to an annealer operating at about650 C. Although the opal phase will generally strike in spontaneously asthe melt is cooled to an amorphous solid, a denser opacity can besecured by heat treating the glass body at about 700-800 C. for about 5minutes to one hour. Inasmuch as this opacifying process is both timeand temperature dependent, longer exposure times will be required atlower temperatures than at higher temperatures to achieve the samedegree of opacity. In any event, nevertheless, the same microstructureof phase separation is observed in the secondarily heat treated articlesas is present in the spontaneously opacified articles.

Table I also includes measurements of various physical properties(Softening Point, Annealing Point, Strain Point, Expansion, and Density)secured on the opalized bodies; these determinations being understakenaccording to conventional procedures. The coefllcients of thermalexpansion lO C.) were measured over the range 0- 300" C.

TABLE I S102, percent 74. 83 73. 34 75. 48 74. 9 73. 95 74. 74. 65 75.60 73. 60 78. 60 77. 60 B203, percent 12.60 12.36 12.88 12. 7 12. 6012.65 11.00 12.00 14.00 9. 00 9. 00 N820, percent 3.05 3.0 ZnO, percent8. 63 8. 46 8.82 8.7 8. 60 8. 0 8. 50 8. 50 8. 50 8. 50 T102, percent-0. 76 0. 75 0. 78 0.80 0. 0 0.80 0. 80 0.80 0.80 A1203, percent 0.05 0.05 0.05 0.05 0. 0. 05 0.05 0.05 0.05 Zl02, percent O. 08 2. 10 0. 05 0.05 0. 05 0. 05 M003, percent 1. 96 L120, percent 1.99 1. 2 1.90 3.003.00 3.00 4. 00 K20, percent 2. 3 Softening point, C 894 980 901 +970978 974 Annealing point, C 622 659 625 636 703 678 709 Strain point, C592 586 562 588 658 617 662 Expansion 28. 1 29. 2 28.0 26.0 36. 8 31.532. 4 31. 8 36. 9 Density 2. 83 2. 311 2.331 2. 337 2. 331 2. 370 2.3312. 324 2. 343 2. 358

ing the cross-sectional microstructure of opal borosilicate glasseshaving compositions approaching, but outside of, the parameters requiredin the present invention.

We have discovered that opal borosilicate glass articles exhibiting avery bright white appearance and greatly improved chemical durability,particularly with respect to resistance to detergent attack, can besecured through the spontaneous opalization and/or the subsequent heattreatment of glass bodies consisting essentially, by weight on the oxidebasis, of 0.5-2.5% L120, 7l0% ZnO, 11-14% B 0 and 71-76% SiO Electronmicroscopy studies of the interior of these articles have shown theopacifying phase to be present as separate droplets, essentiallyspherical in shape, with little or no interconnection. That these rangesof components are critical to the operability of the invention isvisually confirmed in a comparison of FIGS. 3-6 With FIGS. 7-11 as wellas in the results of the detergent solubility test tabulatedhereinafter. It is apparently possible to produce durable borosilicatephase-separated opal glasses, wherein the opacifying material is presentas a discontinuous phase, in compositions falling within theabove-defined, straitly limited ranges without the addition of M00 W0and AS203 to affect the interfacial tension between the glassy matrixand the opacifying phase. These glasses are suitable for manyapplications, e.g., tableware and ovenware, because they combine goodchemical durability with good thermal shock resistance and uniformopacity.

Table I records several examples of glasses falling within and withoutthe compositional parameters of the instant invention in weight percenton the oxide basis. The batch ingredients may comprise any materials,either the oxides or other compounds which, on being melted To determinethe effect of composition changes upon the microstructure of theopalized bodies, replica electron micrographs were made of the crosssection of each example. Hence, FIGS. 1-11 are electron micrographs ofExamples 1-11, respectively. (The white bar at the base of eachphotograph represents one micron.) A study of these photographs clearlyillustrates the substantial efiect which minor compositional changeshave upon the configuration of the separated opal phase. Thus, the opalphase in FIG. 1, a conventional borosilicate opal glass of commerce,assumes a continuous type of structure, i.e., the droplets areinterconnected. In contrast to that configuration, the articles shown inFIGS. 2-6 contain the precipitated phase as very small,spherically-shaped droplets. Example 2 includes the surface tensionagent M00 required in the above-discussed patent application, whereasExamples 3-6 fall within the compositional limitations of the instantinvention. Thus, Examples 3-6 clearly indicate that the presence of suchsurface tension agents is not required to achieve the development of theopal phase in discontinuous droplets (less than 0.5 micron in diameter)where the composition of the glass is included within the narrowparameters of the present invention. FIGS. 7-11 dramatically demonstratethe extreme criticality of the outlined compositional variables of theinstant invention. Hence, even very minor excursions outside of thereported U 0 and B 0 ranges produce the opal phase as interconnectedrather than discontinuous droplets.

The chemical durability of each example recited in Table I with regardto its resistance to detergent attack was measured by subjecting theglasses to strong detergent solutions at elevated temperatures inaccordance with well-known procedures. The specific test employed involved preparing a water solution containing 0.3% by weight of SuperSoilax Brand detergent and immersing the glass samples to be testedtherein while the solution is maintained at 95 C. At two hour intervals,glass samples are removed from the solution, coated with DY- CHEK dye, apenetrating organic liquid, and permitted to stand for five minutes toallow the dye to migrate into the body. Thereafter, the dye is removed,the stain resistance of the glass being graded in accordance with thedifliculty of this removal. Hence, if after standing the dye can beremoved completely with a dry cloth, the glass is categorized AA. Ifremovable with a water soaked cloth, the glass is classed A. Where acloth soaked in detergent solution will remove the stain, the glass isgraded B. If the stain can only be removed by rubbing with a cleansingpowder, the glass is given a C rating and, where unremovable, the glassis classed F.

Table 11 illustrates the correlation between chemical durability and thepresence of the separated phase as very small, discontinuous droplets.Thus, Examples 2-6 exhibit at least a class A stain after 16 hours ofimmersion in the detergent solution whereas Examples 1 and 7-11 failafter 2-8 hours. Here, again, is confirmation of the compositionalrequirement set out to assure the operability of the instant invention.Furthermore, Examples 3-6 demonstrate chemical durability parallelingthat exhibited by Example 2 containing M This factor unequivocally showsthat within the stringent compositional limitations imposed here, aproduct can be manufactured having detergent durability essentially on apar with the products of the above-described application containing M00W0 and AS203.

TABLE II 16 hours 24hours Example 4h0urs Shours compositions to aidmelting and forming, to improve chemical durability, to inhibit unwanteddevitrification, or to modify some other physical property but the totalof all such inclusions must not exceed about 5% by weight, since a finebalance is drawn between the various components to insure thedevelopment of a discontinuous opal phase in very smallspherically-shaped droplets.

Furthermore, the presence of Na O in the glass has been found to beparticularly detrimental to the durability thereof. Therefore, Na O ispreferably absent from the composition although up to about 0.5% byweight can be tolerated. K 0 appears to aid durability but its inclusionin amounts greater than about 3% by weight tends to inhibit opalization,requiring extended secondary heat treatment of the glass to secure adense opacity. The presence of A1 0 improves durability but quantitiesin excess of about 1.5% appear to have an adverse effect uponopalization.

Examples 4 and 5, exhibiting the best resistance to detergent, reflectthe preferred composition ranges of the invention, viz, 1-2% Li O, 89%ZnO, 12-13% B 0 and 73.5-% SiO We claim:

1. A borosilicate opal glass exhibiting a very bright white appearancewith excellent resistance to detergents wherein the opal phase ispresent as discontinuous, spherically-shaped droplets, said glassconsisting essentially by weight on the oxide basis, of about 0.5-2.5 LiO, 7-10% ZnO, 11-14% B 0 and 71-76% SiO the total of Li O, ZnO, B 0 andSiO constituting at least by weight of the composition, and 0-0.5 Na O,03% K 0, and 01.5% A1 0 2. A borosilicate opal glass according to claim1 wherein said glass consists essentially, by weight on the oxide basis,of about 1-2% Li O, 8-9% ZnO, 12-13% B 0 and 73.5-75% SiO ReferencesCited UNITED STATES PATENTS 1,652,259 112/ 1927 Taylor 106---543,275,492 9/1966 Herbert 106-54 3,413,133 11/1968 Stalego 106-541,192,474 1916 Taylor 106-54 OTHER REFERENCES Volf, M. 13., TechnicalGlasses; London, 1961, pp. 28-30.

Ingerson, E., et al.; The Systems K O-ZnO-SiO ZnO-B O SiO and Zn SiO ZnGeO in Amer. .lourn. Sci., 246, 1948, pp. 31-40.

HELEN M. MCCARTHY, Primary Examiner

